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1.
A continuous, 1,420-cm sediment record from Lake Pupuke, Auckland, New Zealand (37°S) was analysed for diatom taxonomy, concentration and flux. A New Zealand freshwater diatom transfer function was applied to infer past pH, electrical conductivity, dissolved reactive phosphorus and chlorophyll a. A precise, mixed-effect regression model of age versus depth was constructed from 11 tephra and 13 radiocarbon dates, with a basal age of 48.2?cal kyr BP. Diatom-inferred changes in paleolimnology and climate corroborate earlier inferences from geochemical analyses (Stephens et al. 2012), with respect to the timing of marked climate changes in the Last Glacial Coldest Phase (LGCP; 28.8?C18.0?cal kyr BP), the Last Glacial Interglacial Transition (LGIT; 18.0 to ca. 12?C10?cal kyr BP) and the Holocene, the onset of which is difficult to discern from LGIT amelioration, but which includes an early climatic optimum (10.2?C8.0?cal kyr BP). The LGCP is readily defined by a reduction in lake level and effective precipitation, whereas the LGIT represents a period of rising lake level, with greater biomass during the Holocene. There was limited change in diatom assemblage structure, influx or inferred water quality during a Late Glacial Reversal (LGR; 14.5?C13.8?cal kyr BP), associated with heightened erosional influx. In contrast, an LGIT peak in paleoproductivity is recorded by increased diatom influx from 13.8 to 12.8?cal kyr BP. Changes in sediment influx and biomass record complex millennial-scale events attuned to the Antarctic Cold Reversal (ACR; 14.5?C12.8?cal kyr BP). Additional millennial-scale environmental change is apparent in the Holocene, with marked changes in lake circulation beginning at 7.6?cal kyr BP, including the onset of seasonal thermal stratification and rapid species turnover at 5.7?cal kyr BP. The most rapid diatom community turnover accompanied widely varying nutrient availability and greater seasonality during the last 3.3?cal kyr. Rising seasonality appears to have been linked to strengthened Southern Westerlies at their northern margins during the middle and late Holocene.  相似文献   

2.
Two cores from Trout Lake, northern Yukon, yielded quantitative estimates of summer air temperatures using fossil midge larvae. Warming began around 14,400?cal?yr BP, with inferred mean July air temperatures reaching values warmer than present by 12,800?cal?yr BP. A 1?°C cooling from 12,200 to 11,200?cal?yr BP closely corresponds with the Younger Dryas chronozone. A broad temperature maximum occurred between 10,800 and 9,800?cal?yr BP, with mean July air temperature about 2.2?°C warmer than present. This represents an early Holocene thermal maximum and coincides with increased organic content of the sediment. Both the shallow- and deep-water cores show similar temperature trends for their overlapping periods. The inferred rise in mean July air temperature at 14,200?cal?yr BP coincides with a shift in vegetation from an herb- to shrub-dominated landscape. In contrast, the increase in Alnus pollen at 6,400?cal?yr BP does not coincide with a change in temperature, but may be a response to a rise in precipitation.  相似文献   

3.
A multi-proxy analysis of two sediment cores from Rantin Lake are used to reconstruct past lake-level changes and to make inferences about millennial-scale variations in precipitation/evaporation (P/E) balance in the southern Yukon, Canada between 10,900 and 3,100?cal?yr BP. Analyses of calcium carbonate and organic matter concentration, magnetic susceptibility, titanium content, dry bulk density, and macrofossils are used to reconstruct water-level changes. The development of sand layers and deformed sediments at the deep-water core site (i.e. Core A-06) prior to ~10,900?cal?yr BP suggest that lake level was lower at this time. Fine-grained organic sediment deposited from 10,600 to 9,500?cal?yr BP indicates a rise in lake level. The formation of an unconformity at the shallow cores site (Core C-06) and the deposition of shallow-water calcium carbonate-rich facies at the Core A-06 site between ~9,500 and ~8,500?cal?yr BP suggest lower lake levels at this time. Shallow-water facies gradually transition into a sand layer that likely represents shoreline reworking during an extreme lowstand that occurred at ~8,400?cal?yr BP. Following this low water level, fine-grained organic-rich sediment formed by ~8,200?cal?yr BP, suggesting deeper water conditions at core site A-06. Calcium carbonate concentrations are relatively low in sediment deposited from ~6,300 to 3,100?cal?yr BP in Core A-06, indicating that lake level was comparatively higher during the middle and late Holocene. In general, results from this study suggest that the early Holocene was characterized by high P/E from ~10,500 to 9,500?cal?yr BP, low P/E from ~9,500 to 8,400?cal?yr BP, and return to higher P/E from ~8,200 to 3,100?cal?yr BP.  相似文献   

4.
We present a record of Holocene environmental change on the east coast of Korea, inferred using pollen, carbon-isotopic composition (??13C), total organic carbon, total sulfur, carbon/nitrogen ratios, particle size analysis and major element geochemistry in a sediment core from Soonpogae Lagoon. A multi-proxy paleoenvironmental approach had not previously been applied in Korea and allowed us to reconstruct climate and vegetation change, sea-level rise, lagoon development, and human impact on the east coast of Korea over the last 8,000?years. Evidence from Soonpogae Lagoon supports the following three conclusions: (1) As a drying trend prevailed on the east coast of Korea after ~5,900?cal?yr BP, chemical weathering weakened and herbaceous plants became more important than during the previous humid phase (Holocene Climate Optimum), (2) Sea-level rise on the east coast slowed dramatically about 6,800?cal?yr BP, resulting in low rates of sedimentation in Soonpogae Lagoon, and (3) Soonpogae Lagoon was almost completely isolated from the sea by sand barriers when human impact intensified ~2,100 BP.  相似文献   

5.
Lacustrine sediments in north-eastern Germany have rarely been used as archives to address the effects of climate change and human impact on both lake ecosystem and landscape evolution for this region. Sacrower See, a hardwater lake located in Brandenburg, provides a unique sediment record covering the past 13,000 years which was used to reconstruct climatic and anthropogenic forcing on lacustrine sedimentation. Time control is provided by 12 AMS 14C dates of terrestrial plant remains, the Laacher See Tephra, and the onset of varve formation in AD 1870 (80 cal. BP). Geochemical (including XRF logging of major elements, CNS analyses as well as δ13Corg and δ15N measurements) and pollen analyses allowed detecting detailed environmental changes in the sediment record. During the Younger Dryas cold phase increased soil erosion and hypolimnetic oxygen depletion enhanced the nutrient supply to the lake water causing eutrophic conditions. The beginning of the Holocene is characterized by large changes in C/N ratios, total sulphur, δ13C of bulk organic matter as well as in K, Si, and Ti, reflecting the response of the lake’s catchment to climatic warming. Reforestation reduced the influx of detrital particles and terrestrial organic matter. The first, rather weak evidence of human impact is documented only in the pollen record at 5,500 cal. BP. However, until 3,200 cal. BP sedimentological and geochemical parameters indicate relatively stable environmental conditions. During periods of intense human impact at around 3,200, 2,800, and 900 cal. BP peaks in Ti and K represent phases of increased soil erosion due to forest clearing during the Bronze Age, Iron Age, and Medieval Times, respectively. In general, greater variation is observed in most variables during these perturbations, indicating less stable environmental conditions. The steady rise of biogenic silica accumulation rates during the Holocene reflects an increasing productivity of Sacrower See until diatoms were outcompeted by other algae during the last centuries. The applied multi-proxy approach fosters the interpretation of the sediment record to reveal a consistent picture of environmental change including environmental factors controlling lake ontogeny and the effects of human impact.  相似文献   

6.
Geochemical data obtained from X-ray fluorescence, physical properties, total organic and inorganic carbon content (TOC/TIC), and diatom analysis from a 6.61-m-long sedimentary sequence near the modern northern shore of Lake Zirahuen (101° 44′ W, 19° 26′ N, 2000 m asl) provide a reconstruction of lacustrine sedimentation during the last approximately 17 cal kyr BP. A time scale is based on ten AMS 14C dates and by tephra layers from Jorullo (AD 1759-1764) and Paricutin (AD 1943-1952) volcanoes. The multiproxy analyses presented in this study reveal abrupt changes in environmental and climatic conditions. The results are compared to the paleo-record from nearby Lake Patzcuaro. Dry conditions and low lake level are inferred in the late Pleistocene until ca. 15 cal kyr BP, followed by a slight but sustained increase in lake level, as well as a higher productivity, peaking at ca. 12.1 cal kyr BP. This interpretation is consistent with several regional climatic reconstructions in central Mexico, but it is in opposition to record from Lake Patzcuaro. A sediment hiatus bracketed between 12.1 and 7.2 cal kyr BP suggests a drop in lake level in response to a dry early Holocene. A deeper, more eutrophic and turbid lake is recorded after 7.2 cal kyr BP. Lake level at the coring site during the mid Holocene is considered the highest for the past 17 cal kyr BP. The emplacement of the La Magueyera lava flows (LMLF), dated by thermoluminiscence at 6560 ± 950 year, may have reduced basin volume and contributed to the relative deepening of the lake after 7.2 cal kyr BP. The late Holocene (after 3.9 cal kyr BP) climate is characterized by high instability. Extensive erosion, lower lake levels, dry conditions and pulses of high sediment influx due to high rainfall are inferred for this time. Further decrease in lake level and increased erosion are recorded after ca. AD 1050, at the peak of Purepechas occupation (AD 1300–1521), and until the eighteenth century. Few lacustrine records extend back to the late Pleistocene—early Holocene in central Mexico; this paper contributes to the understanding of late Pleistocene-Holocene paleoclimates in this region.  相似文献   

7.
Postglacial sediments from Lake Jues, located at the SW margin of the Harz Mountains, were investigated for pollen, diatoms and sediment characteristics. The paper focuses on the time period between 7600 and 1200 y cal. BP (calibrated years before present), during which human impact began to influence the environnment of the lake. Climate variability was mainly inferred from sediment characteristics and changes in algal assemblages. The observed climatic changes coincide well with those recorded from other sites. Neolithic settlement started during the warm and dry Atlantic period. Intensive land use occurred under favourable climatic conditions during the Bronze and the Iron Ages, while human activities declined during cool and wet periods around 4000 and 2700 y cal. BP and after 2300 y cal. BP. The study shows that climate strongly influenced human settlement at remote locations like Lake Jues.  相似文献   

8.
During recent years, numerous studies dealing with Holocene lake level fluctuations have been conducted in Finnish Lapland. However, no quantification of lake level variations exists to date. Here, we applied a recently developed modern cladocera – lake depth transfer model to subfossil cladocerans analysed from three small and shallow (< 6 m) kettle-hole lakes in northwestern Finnish Lapland to provide estimates of the amplitudes of long-term lake-level changes in the region. The quantitative inferences were compared to pollen, charcoal and geochemical records from one of the study sites. The lake levels were inferred to be high during the early Holocene; they faced marked reduction up to 4–6 m in the mid-Holocene (≈7000–4000 cal yr BP), and rose again during the latter part of the Holocene. There is some indication of lowered lake levels around 1500 cal yr BP, but interpretation of such small-scale changes is hazardous due to large prediction errors in the initial cladoceran model. The overall pattern of the Holocene lake level variation generally followed the regional changes in climate humidity as reconstructed in previous studies by means of other sedimentary proxy indicators, such as pollen and oxygen isotopic compositions. We postulate that changes in winter precipitation may have had a greater influence on lake-levels than variations in summer precipitation or evaporation.  相似文献   

9.
Charcoal preserved in lake sediments is commonly used to reconstruct past trends in fire occurrence. However, interpretation of the charcoal record is often complicated, as changes in charcoal influx could represent natural shifts in fire regimes associated with changes in climate, changes in vegetation, or changes in patterns of anthropogenic burning. Here we examine sedimentary charcoal records from three lakes on the Caribbean islands of Hispaniola and Puerto Rico: Laguna Saladilla in the Dominican Republic, Lake Miragoane in Haiti, and Laguna Tortuguero in Puerto Rico. All records are based on microscopic charcoal fragments quantified from pollen slides and cover the last 7,000 or more years of the Holocene. We compare charcoal influx values to archeological and palynological evidence of human activity and explore the role of increasing winter insolation over the Holocene in driving increased charcoal deposition beginning ca. 6,000–5,000 cal yr BP. An increase in charcoal influx at Laguna Tortuguero at ca. 5,200 cal yr BP, previously interpreted as a signal of human settlement predating archeological evidence, may instead reflect insolation-driven shifts in winter drying that led to more frequent and possibly more intense natural fires. Decreased charcoal influx around 3,200 cal yr BP may signal human modification of the environment that altered fire frequency and/or intensity. Comparing the records from these three lowland Caribbean sites highlights possible intervals of synchronous, climate-driven burning as distinct from more localized anthropogenic burning.  相似文献   

10.
This study used organic matter in oligotrophic Lake Constance (southern Germany) to reconstruct lake environment and to disentangle the multiple factors, such as climate change and human impacts, which influence sedimentation in large lakes. A sediment core from Upper Lake Constance, which represents 16,000 years of Late Glacial and Holocene lake history, was analysed for organic biomarkers, hydrogen index and elements calcium, strontium, and magnesium. Magnetic susceptibility was measured to establish a high-resolution stratigraphic framework for the core and to obtain further information about changes with respect to relative allochthonous versus autochthonous sedimentation. Dinosterol—a biomarker for dinoflagellates—and calcium have low concentrations in Younger Dryas sediments and consistently high concentrations between 10,500 and 7,000 cal. years BP. These variations are attributed to changes in lake productivity, but are not reflected in the proportion of total organic carbon within the sediment. During the Younger Dryas and between 6,000 and 2,800 cal. years BP, concentrations and accumulation rates of land-plant-derived C29-steroids (β-sitosterol, stigmastanol and stigmasterol), in combination with a relatively low HI, indicate periods of enhanced terrigenous input to the lake. For the Younger Dryas, higher runoff can be attributed to a cold climate, leading to decreased vegetation cover and increased erosion. After 6,000 cal. years BP, high terrestrial input may be explained by enhanced precipitation. Biomarker and HI results, in combination with archaeological studies, raise the question as to whether lakeshore settlements affected sedimentation in Upper Lake Constance between 6,000 and 2,800 cal. years BP.  相似文献   

11.
Small lakes and wetlands from high elevation within the Sierra Nevada Range (southern Spain) preserve a complete post-glacial Holocene record. Isotopic, TOC and C/N analyses, carried out on a sediment core, show various stages in the evolution of the Borreguiles de la Virgen, which today constitute a small bog at about 2,950?m above sea level. Glacial erosion generated a cirque depression, which became a small lake during the first phase of infilling (from?8,200 to 5,100?cal?yr BP), as suggested by sedimentary evidence, including an atomic C/N ratio generally below 20, low TOC values and the highest ??13C and ??15N values of the record. These results imply significant algal productivity, which is confirmed by the microscopic algal remains. Drier conditions became established progressively in this area from?5,100 to 3,700?cal?yr BP. Subsequently, the lake evolved into a bog as shown by geochemical evidence (C/N ratios above 20, high TOC content and low ??13C values). Unstable conditions prevailed from?3,600 to 700?cal?yr BP; an extremely low sedimentation rate and scarcity of data from this period do not allow us to make a coherent interpretation. Fluctuating conditions were recorded during the last?~700?cal?yr BP, with wetter conditions prevailing during the first part of the interval (with C/N rate below 20) up to 350?years ago. In general, a gradual trend toward more arid conditions occurred since?~6,900?cal?yr BP, with a further increase in aridity since?~5,100?cal?yr BP. This evidence is consistent with other contemporaneous peri-Mediterranean records.  相似文献   

12.
Three lake sediment sequences (lakes Nero, Chashnitsy, Zaozer’e) from the Rostov-Jaroslavl’ region north of Moscow were studied to provide information on palaeoclimatic and palaeoenvironmental changes during the past 15,000 cal yr. The multi-proxy study (i.e., pollen, macrofossils, mineral magnetic measurements, total carbon, nitrogen and sulphur) is chronologically constrained by AMS 14C measurements. Lake Nero provided the longest sedimentary record back to ca. 15,000 cal yr BP, while sediment accumulation began around ca. 11,000 cal yr BP in the two other lakes, possibly due to melting of permafrost. Limnic plant macrofossil remains suggest increased lake productivity and higher mean summer temperatures after 14,500 cal yr BP. While the late glacial vegetation was dominated by Betula and Salix shrubs and various herbs, it appears that Betula sect. Albae became established as early as 14,000 cal yr BP. Major hydrological changes in the region led to distinctly lower lake levels, starting 13,000 cal yr BP in Lake Nero and ca. 9000 cal yr BP in lakes Chashnitsy and Zaozer’e, which are situated at higher elevations. These changes resulted in sedimentary hiatuses in all three lakes that lasted 3500–4500 cal yr. Mixed broad-leaved – coniferous forests were widespread in the area between 8200 and 6100 cal yr BP and developed into dense, species-rich forests between 6100 and 2500 cal yr BP, during what was likely the warmest interval of the studied sequences. Agricultural activity is documented since 500 cal yr BP, but probably began earlier, since Rostov was a major capital by 862 A.D. This apparent gap may be caused by additional sedimentary hiatuses around 2500 and 500 cal yr BP.  相似文献   

13.
Changes in the diatom assemblages preserved in a sediment core taken from a small lake located north of arctic treeline on the western Taimyr Peninsula, Russia, were examined in order to investigate late Holocene (i.e., ca 5000 cal yr BP to present) climatic and environmental changes within the region. Early diatom assemblages were dominated by benthic Fragilaria taxa and indicate a transitional phase in the lake history, most likely reflecting lake development and environmental change associated with treeline retreat to the south of the study site. Concurrent with pollen and macrofossil evidence of a vegetation shift to shrub tundra in the catchment basin at ca 4200 cal yr BP, an increase in cold-water taxa, followed by little change in diatom assemblages until ca 2800 cal yr BP, suggests that conditions were relatively cool and stable at this time. The last 2000 years of the Middendorf Lake record have been marked by fluctuating limnological conditions, characterized by striking successional shifts between Fragilaria pinnata and Aulacoseira distans var. humilis. Recent conditions in Middendorf Lake indicate an increase in diatom taxa previously rare in the record, possibly associated with twentieth-century climatic warming. The Middendorf Lake record indicates that significant limnological change may occur in the absence of catchment vegetation shifts, suggesting late-Holocene decoupling of aquatic and terrestrial responses to climatic and hydrological change. Our study results represent one of the few paleoecological records currently available from northern Russia, and highlight the need for further development of calibration data sets from this region.  相似文献   

14.
A late Quaternary diatom stratigraphy of Lago Puyehue (40°40′ S, 72°28′ W) was examined in order to infer past limnological and climatic changes in the South-Chilean Lake District. The diatom assemblages were well preserved in a 1,122 cm long, 14C-dated sediment core spanning the last 17,900 years, and were in support of an early deglaciation of Lago Puyehue. The presence of a short cold spell in South Chile, equivalent to the Younger Dryas event in the Northern Hemisphere, the Antarctic Cold Reversal in Antarctica, or the Huelmo-Mascardi event in southern South America, was not clearly evidenced in the diatom data, although some climate instability may have occurred between 13,400 and 11,700 cal. yr. BP, and a relatively long period (between 16,850 and 12,810 cal. yr. BP) with low absolute abundances and biovolumes could be tentatively interpreted as a period of low rainfall and/or temperatures. An increase in the moisture supply to the lake was tentatively inferred at 12,810 cal. yr. BP. After 9,550 cal. yr. BP, inferred stronger and longer persisting summer stratification, may have been the result of the higher temperatures associated with an early-Holocene thermal optimum. The mid-Holocene appeared to be characterized by a decrease in precipitation, culminating around 5,000 cal. yr. BP, and rising again after 3,000 cal. yr. BP, likely associated with a previously documented lowered frequency and amplitude of El Niño events. An increase in precipitation during the late Holocene (3,000 cal. yr. BP–present) might have marked subsequent increased frequency of El Niño occurrences, leading to drier summers and slightly moister winters in the area.  相似文献   

15.
Lacustrine records from the northern margin of the East Asian monsoon generate a conflicting picture of Holocene monsoonal precipitation change. To seek an integrated view of East Asian monsoon variability during the Holocene, an 8.5-m-long sediment core recovered in the depocenter of Dali Lake in central-eastern Inner Mongolia was analyzed at 1-cm intervals for total organic and inorganic carbon concentrations. The data indicate that Dali Lake reached its highest level during the early Holocene (11,500–7,600 cal yr BP). The middle Holocene (7,600–3,450 cal yr BP) was characterized by dramatic fluctuations in the lake level with three intervals of lower lake stands occurring 6,600–5,850, 5,100–4,850 and 4,450–3,750 cal yr BP, respectively. During the late Holocene (3,450 cal yr BP to present), the lake displayed a general shrinking trend with the lowest levels at three episodes of 3,150–2,650, 1,650–1,150 and 550–200 cal yr BP. We infer that the expansion of the lake during the early Holocene would have resulted from the input of the snow/ice melt, rather than the monsoonal precipitation, in response to the increase in summer solar radiation in the Northern Hemisphere. We also interpret the rise in the lake level since ca. 7,600 cal yr BP as closely related to increased monsoonal precipitation over the lake region resulting from increased temperature and size of the Western Pacific Warm Pool and a westward shifted and strengthened Kuroshio Current in the western Pacific. Moreover, high variability of the East Asian monsoon climate since 7,600 cal yr BP, marked by large fluctuations in the lake level, might have been directly associated with variations in the intensity and frequency of the El Niño-Southern Oscillation (ENSO) events.  相似文献   

16.
A 2.5-m-long sediment core was retrieved from Lake Somaslampi, a small lake located in a kame field on the north slope of the Scandes Mountains in Finnish Lapland. Holocene environmental changes were inferred from the lithological, geochemical, pollen, diatom and Cladocera records stored in the lake sediment. The chronology was based on six radiocarbon AMS dates supported by a palynological control chronology. The sediment profile consists of a glacial sedimentary sequence truncated by a lacustrine one. A hiatus, tentatively correlated with climate cooling and advances of glaciers during the 8.2 ka yrs BP “Finse cooling Event”, occurs between these sequences. The glacial sequence was composed of fluvioglacial clastics, smoothly changing into glacio-lacustrine diatomaceous ooze deposited in a meromictic proglacial lake that covered the kame field. The meromixis was probably caused by the greater depth of the lake, the extended ice-cover, and the microbial mats covering large areas of the lake bottom. A distinct change in the biota of the glacio-lacustrine sediments indicates higher trophic conditions than during deposition of the fluvioglacial clastics. The late-Pleistocene vegetation was characterised by subarctic birch tundra vegetation (Betula–Salix–Ericaceae) with low biodiversity gradually changing to Betula–Pinus dominance in the early Holocene. The lake was deep and had a diatom inferred pH ~ 7 indicated also by the dominance of planktonic Cladocera. The base of the lacustrine sediment sequence (6,650–6,300 cal. BP) consisted of loess-rich sediment indicating an increase in eolian activity. This is also supported by the pollen record, which is dominated by more long-distant taxa such as Alnus and Pinus, and by the increased C/N ratio of the sediment. After the initial meromictic phase of the lake, an abrupt lowering of the water level occurred. Lake Somaslampi was isolated from the larger Pre-Lake Somas basin and became holomictic, shallow, much warmer and more productive, until the deterioration of climate around 3,000 yr BP and the increased input of clastics from the tundra soils. The vegetation followed the general climatic trend by gradually changing from the dominance of Betula and Pinus to the dominance of more tundra-related vegetation like Poaceae and Cyperaceae. However, the higher frequencies of planktonic Cladocera and centric diatoms in the most recent sediments indicates higher trophic conditions, increased turbulence and a prolonged ice-free period, which can possibly be linked to the recent climate warming especially in areas of higher altitude and latitude.  相似文献   

17.
Analyses of lithology, organic-matter content, magnetic susceptibility, and pollen in a sediment core from Okpilak Lake, located in the northeastern Brooks Range, provide new insights into the history of climate, landscape processes, and vegetation in northern Alaska since 14,500?cal?year BP. The late-glacial interval (>11,600?cal?year BP) featured sparse vegetation cover and the erosion of minerogenic sediment into the lake from nearby hillslopes, as evidenced by Cyperaceae-dominated pollen assemblages and high magnetic susceptibility (MS) values. Betula expanded in the early Holocene (11,600?C8,500?cal?year BP), reducing mass wasting on the landscape, as reflected by lower MS. Holocene sediments contain a series of silt- and clay-dominated layers, and given their physical characteristics and the topographic setting of the lake on the braided outwash plain of the Okpilak River, the inorganic layers are interpreted as rapidly deposited fluvial sediments, likely associated with intervals of river aggradation, changes in channel planform, and periodic overbank flow via a channel that connects the river and lake. The episodes of fluvial dynamics and aggradation appear to have been related to regional environmental variability, including a period of glacial retreat during the early Holocene, as well as glacial advances in the middle Holocene (5,500?C5,200?cal?year BP) and during the Little Ice Age (500?C400?cal?year BP). The rapid deposition of multiple inorganic layers during the early Holocene, including thick layers at 10,900?C10,000 and 9,400?C9,200?cal?year BP, suggests that it was a particularly dynamic interval of fluvial activity and landscape change.  相似文献   

18.
Paleohydrology studies at Mathews Pond and Whitehead Lake in northern Maine revealed synchronous changes in lake levels from about 12,000 14C yrs BP to the present. We analyzed gross sediment structure, organic and carbonate content, mineral grain size, and macrofossils of six cores from each of the two lakes, and obtained 72 radiocarbon dates. Interpretation of this paleo-environmental data suggests that the late-glacial and Younger Dryas climate was dry, and lake levels were low. Early Holocene lake levels were considerably higher but declined for an interval from about 8000 to 7200 14C yrs BP. Sediment of both lakes contains evidence of a dry period at ∼7400 14C yrs BP (8200 cal yr). Lake levels of both sites declined abruptly about 4800 14C yrs BP and remained low until 3000 14C yrs BP. Modern lake levels were achieved only within the past 600 years. The west-to-east, time-transgressive nature of lake-level changes from several sites across northeastern North America suggests periodic changes in atmospheric circulation patterns as a driving force behind observed moisture balance changes. Electronic supplementary material to this article is available at and accessible for authorized users.  相似文献   

19.
We investigated oxygen and carbon isotopes of bulk carbonate and of benthic freshwater ostracods (Candona candida) in a sediment core of Lago Piccolo di Avigliana that was previously analyzed for pollen and loss-on-ignition, in order to reconstruct environmental changes during the late glacial and early Holocene. The depth–age relationship of the sediment core was established using 14 AMS 14C dates and the Laacher See Tephra. While stable isotopes of bulk carbonates may have been affected by detrital input and, therefore, only indirectly reflect climatic changes, isotopes measured on ostracod shells provide unambiguous evidence for major environmental changes. Oxygen isotope ratios of ostracod shells (δ18OC) increased by ～6‰ at the onset of the Bølling (～14,650 cal BP) and were ～2‰ lower during the Younger Dryas (～12,850 to 11,650 cal BP), indicating a temporal pattern of climate changes similar to the North Atlantic region. However, in contrast to records in that region, δ18OC gradually decreased during the early Holocene, suggesting that compared to the Younger Dryas more humid conditions occurred and that the lake received gradually increasing input of 18O-depleted groundwater or river water.  相似文献   

20.
ABSTRACT. Two well dated Holocene sediment records bordering the Denmark Strait region have been used to reconstruct past climate variability. The content of biogenic silica, classic and organic material and moss in a lacustrine record from Lake N14 has been used to infer past variability in precipitation and temperature in southern Greenland. Sedimentologic and petrologic composition of sand in a shelf sediment record from the Djúpáll trough is used to infer past variability in the northwestern storm activity on northwestern Iceland, which probably also affected the inflow of polar waters from the East Greenland Current. Our evaluation of these records with a number of previous studies from the region documents Holocene climatic optimum conditions peaking between 8000 and 6500 calendar years before present (cal yr BP). Mid-Holocene climate deterioration set in around 5000 cal yr BP followed by a further marked setback around 3500 cal yr BP. A stacking of climate variability on a centennial timescale from previous studies in the area shows a fairly good correspondence to the timing of marked cold and warm events as evidenced from the Lake N14 and the Djúpáll trough records. Cooler periods are explained as the response to marked incursions of ice-laden polar water from the Arctic Ocean to the Denmark Strait region. Cool northerly and northwesterly winds along the East Greenland coast in relation to frequent strong atmospheric low pressure in the Barents Sea, coupled with strong high pressure over Greenland, would have favoured southward export of polar waters. A comparison with the proxy records of nuclide production (14C and 10Be) suggests that solar activity may have had some influence on the atmospheric pressure distribution in the Denmark Strait region.  相似文献   

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